Method and apparatus for return beam readout through a matrix electron lens

ABSTRACT

An electron beam-addressable memory apparatus and method for writing information on a memory target and reading information stored thereon are disclosed. Readout of stored information is achieved by scanning a primary electron beam across the target and detecting the secondary electrons emitted therefrom. The primary electron beam is directed first to a specific lenslet of a matrix electron lens by a coarse deflection assembly and then to a precise location on the target by a fine deflection assembly. Secondary emission electrons emanating from the surface of the target as a result of the primary electron beam are focussed back to the matrix electron lens and then are directed along a trajectory substantially similar to that of the primary electron beam to an electron detector. Since the number of detected electrons is modulated by the information stored on the target, information representative of digital data is conveniently stored and efficiently read out from the memory.

[ June 28, 1974 United States Patent [191 Hughes METHOD AND APPARATUS FOR RETURN Primary Examiner-Archie R. Borchelt BEAM READOUT THROUGH A MATRIX Assistant Examiner-Saxfield Chatmon, Jr. ELECTRON LENS Att0mey,-Agent, 0r Firm-Paul F. Wille; Joseph T. Cohen; Jerome C. Squillaro [75] Inventor: William C. Hughes, Scotia, N.Y.

Assignee: General Electric Company,

Schenectady, NY.

Oct. 2, 1972 [22] Filed:

method for writing information on a memory target [21] APPL 294,018 and reading information stored thereon are disclosed.

Readout of stored information 18 achieved by scanning a primary electron beam across the target and detecting the secondary electrons emitted therefrom. The primary electron beam is directed first to a specific lenslet of a matrix electron lens by a coarse deflection assembly and then to a precise location on thetarget 7. 313/432 I-I0lj 151/48 313/68, 68 D, 92 PD, 84,

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by a fine deflection assembly. Secondary emission References Cited electrons emanating from the surface of the target as a UNITED STATES PATENTS result of the primary electron beam are focussed back to the matrix electron lens and then are directed along a trajectory substantially similar to that of the primary electron beam to an electron detector. Since the number of detected electrons is modulated by the information stored on the target, infonnation representative of digital data is conveniently stored and efficiently read out from'the memory.v

RD 88 mm 33 an 2,770,747 Jensen..............'.....

2,916,661 Davis...............

2,922,071 1/1960 Hergenrother...

3,281,621 10/1966 Clayton..............

FOREIGN PATENTS OR APPLICATIONS 248,974 1/1964 Australia........................... 313/68 R 14 Claims, 4 Drawing figures I 33 80LENO/D llllllllllllIlllllIlllllll[I'lllllIlIIlIllllllIlllllllll'HlIlllIllllllllllllll lIIIlllllllllllllllllllllllllllllllltlllllllllllllllll llllllllllllllllllll FINE DEFL EC MR COURSE DEFL EC TOR RETURN BEA/'7 llIllllllllllll'lllllllllllllllllllllllIlIllllllllllllTllHlIIllllllllltllllll llllllllllllllllllLlllIlllllll'llllllllIllllllltHllllllllllllllllllllllllLL CONDENSER L ENS TFE 601V osrscran BEAM CE TERI/V6 PATENIEBmze I974 SHEEI 1 UF 2 METHOD AND APPARATUS FOR RETURN BEAM READOUT THROUGH A MATRIX ELECTRON LENS portant. Advances in computer technology and in the field of automation have placed increasing demands on data storage devices. Presently available data storage devices such as magnetic tapes and magnetic drums have a maximum storage density of the order of 1,000 bits per centimeter squared and consequently require approximately 1,000 square centimeters of storge for storing bits of data. Such large storage areas substantially increase the access time required to read out such data. Hence, researchers have been attempting to find new storage mechanisms with higher density and shorter access times.

US. Pat. No. 3,170,083 to Newberry describes a high density, fast access time data storage system employing a charged particle beam-addressed memory system. It has been found that chaged particle beams, such as electron beams, can be focussed to spot'diameters in the order of 100 Angstroms or less, thereby providing a means for storing data in storage sites having dimensions of a few 100 A. Such data storage systems have the potential of providing in excess of 10 storage sites per square centimeter. Accordingly, charged particle beam-addressed memories offer a possible solution to the ever-increasing demands of the computer industry for higher density storage media.

Read out of information stored in the memory target depends upon the type of target employed. For example, some storage targets provide a different number of reflected electrons, depending upon the presence or absence of information stored thereon. Other storage targets'detect the current flow through the target and differences therein as a result of information stored thereon. A particularly desirable method for detecting information stored on a memory target is to measure prises metal removed and metal remaining portions;

the number of secondary electrons emitted from the surface of the storage target differs when the electron beam is incident on a metal removed area from when the electron beam is incident on a metal remaining area. By detecting this difference in secondary electron emission, it is possible to read out information stored on the storage target.

One of the difficulties encountered in measuring or detecting this difference in secondary electron emission is the location of the detector. The detector must be located outside of the field of the primary electron beam but sufficiently close to the target to detect the secondary electron emission. Attempts to provide such detectors have not been entirely satisfactory because the presence of the electron optical system severely limits the location of such detectors. As a result, signal detection and signal-to-noise ratios are generally unacc'eptable.

It is, therefore, an object of this invention to provide an electron beam-addressable memory system with an efficient secondary electron emission detection apparatus.

It is a further object of this invention to provide a method and apparatus for collecting signal electrons from a target structure with a simple and efficient electron detection apparatus.

Another object is to provide data storage and readout apparatus having high storage density employing an electron beam for both reading and writing.

Still another object is to provide a method and apparatus for accurately positioning a primary electron beam on a memory target and for collecting signal electrons from the target as a result of the primary beam scanning the target.

Briefly, in accord with my invention, a new and improved method and apparatus is provided for reading out information stored on a memory target in the form of secondary electrons emitted from the memory surface as a result of a scanning primary electron beam. The primary electron beam is directed to the memory target by a coarse deflection system positioned along the path of the electron beam for coarsely directing the electron beam to a selected lenslet of a matrix lens. The electron beam exiting from the matrix lens is then finely deflected toimpinge .on the target surface precisely at the desired point of impingement and-normal thereto. Secondary emission electrons emanating from the surface of the memory structure as a result of the primary electron beam impinging thereon are focussed by the fine deflector back to the matrix lens along a trajectory substantially similar to that of the primary electron beam to a return beam detector where the secondary electrons are detected. Since the number of detected electons is proportional to the number emitted from the surface of the memory target, by appropri-- ately selecting the memory target such that different numbers of electrons are emitted from a storagesite representing a logic zero from a storage site representing a logic one,'digital data is conveniently and efficiently read out from the memory target.

- The features of the invention believed to be novel are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may be best understood by reference to the following description taken in conjunction with the accompany.- ing drawings in which:

FIG. 1 is a simplified sectional view of apparatus embodying the electron beam memory system of the invention;

FIG. 2 schematically illustrates the details of a matrix electron lens used in accord with my invention;

FIG. 3 schematically illustrates the paths of the primary and secondary electrons between the matrix electron lens and the target; and

FIG. 4 is a plot of spherical aberration of the matrix electron lens versus lenslet radius for various lens plate spacings.

FIG. 1 schematically illustrates a data'storage appa- 'ratus l0 employing a return beam matrix lens optical system for readout of the stored information in accord with the principles of the instant invention. The data storage apparatus 10 comprises an electron source or gun 11 for projecting a beam of electrons 12 to a memsource 11 is collimated by a magnetic condenser lens 14 which modifies the electron trajectories during their passage through the condenser lens 14 so as to provide a substantially parallel beam of electrons. The electron beam 12 is accelerated by an accelerator 15 having minimum lens characteristics. The beam of electrons is then centered along a selected axis by the use of suitable beam centering apparatus 16 Comprising xand y-centering coils, 17 and 18, respectively.

The electron beam 12 then enters one end of a coarse deflection region comprising a horizontal and a vertical deflection assembly 20, illustrated in FIG. 1 as magnetic deflection coils 21 and 22, respectively. the horizontal and vertical deflection assembly 20 deflects the electron beam 12 in a predetermined manner and sequence both in the horizontal and vertical direction to scan the beam across a matrix electron lens 23. The matrix electron lens 23 comprises two parallel lens plates 24 and 25 spaced apart from each other by approximately 20 to 80 mils, for example, and having a plurality of aligned apertures therein. By way of example and without limiting the present invention, the size of the apertures may range between approximately 5 and 50mils, for example.

FIG. '2 illustrates more clearly the details of the matrix electron lens 23 with lens plates 24 and 25 having a plurality of aligned apertures or lenslets 26 through which the electron beam is directed. The lens plates 24 and 25 are connected to suitable voltage sources to produce a decelerating effect on the electronbeam as it passes therethrough.

Returning again to F IG, 1, the electron beam, after passing througha selected lenslet of the matrix electron lens 23, is further decelerated by an electric field established by a decelerator 28 positioned between the matrix electron lens 23 and the storage target 13. Surrounding the decelerator 28 is a fine deflection assembly 30 which provides fine horizontal and vertical deflection of the electron beam.

operationally, the electron beam 12 is directed first to a specific lenslet 26 by the horizontal and vertical ately in front of the particular lenslet through which it passes. FIG. 2 illustrates the focal point by the numeral 32. The focal point is then relayed to the target surface with a single node, under the action of an axial magnetic field produced by a solenoid 33 (illustrated in FIG. 1) which surrounds the coarse deflector 20, the matrix lens 23, the fine deflector 30 and the target structure 13.

The target structure 13 may comprise a semiconductor wafer having a pattern of radiation absorbing regions coated over its incident radiation receiving surface and which is interrogated by impingement of an electron beam on the surface thereof, such as is described in U.S. Pat. No. 3,550,094. Altemately, the

storage target may comprise a dielectric layer'and a juxtaposed semiconductor layer sandwiched between a tin oxide electrode and an electron transparent aluminum electrode, which exhibits localized breakdown in the dielectric layer when the semiconductor layer is struck by a writing electron beam. Readout is then achieved by detecting secondary electron emission from the storage target by impingement of an electron beam. Such a targetstructure is described in U.S. Pat. No. 3,573,753. Still further, the target structure may comprise an amorphous semiconductor material which is rendered crystalline in selected regions by a writing electron beam. Read-outisachieved by detecting secondary electron emission, which differs for crystalline v and amorphous semiconductor material. Such a target structure is described in U.S. Pat. No. 3,750,117.

In accord with my invention, the secondary emission electrons are returned back through the matrix electron lens 23 along a path substantially similar to that deflection assembly 20 and then further directed to a specific location on the storage target 13 by the fine deflection assembly 30. Thus, once the coarse deflection assembly 20 has deflected the electron beam onto any particular lenslet 26, the electron beam may be precisely controlled and directed to a specific location on the target structure 13 by the fine deflection assembly taken by the primary electron beam 12 where they are detected by a detector 35. Under the influence of the axial magnetic field of the solenoid 33 and the magnetic deflection field produced by the fine deflection assembly 30, along with the decelerating electric field produced by the decelerator 28, the secondary electrons are drawn back to the aperture in the electron lens 23 through which the primary beam has passed. Whereas the decelerating electric field produced by the decelerator 28 has only a minor effect on the primary electron beam, the low energy secondary electrons are accelerated substantially by this field and hence the secondary electrons are focussed back to the electron lens 23.

FIG. 3 illustrates the return of secondary electrons from the target surface to the electron lens .23 along paths 36 and 37. As is illustrated, the secondary electrons are guided and focussed by the magnetic field so that they return to the same lenslet (through which the primary electron beam passed) in the electron lens 23 with a multi-node focus of varying pitch. This occurs even though the primary electron beam is deflected by the fine deflection assembly 30. By adjusting the accelerating electric field, the final focus node for the secondary electrons is made to coincide with the return focal point of the electron lens. The secondary electrons therefore are collimated and further accelerated by the electron lens and exit from the electron lens along a path similar to that followed by the primary electron beam 12.

The return beam of secondary electrons is directed and guided by the magnetic field back to the detector 35. The detector 35 is provided with an aperture which is just large enough to allow the primary electron beam to pass through, but since the return beam diameter is considerably larger than the primary beam, most of the return beam electrons are collected by the detector 35. The detector 35 is preferably a semiconductor particle detector having substantial current gain (e.g., 100 1,000 times the secondary electron current) so that an electrical output signal having an amplitude proportional to the magnitude of the return beam secondary electrons is provided.

Having thus described generally a data storage system employing return beam readout, a more detailed discussion of the electron optics will now be presented to enable those skilled in the art to better appreciate the numerous advantages associated with data storage systems constructed in accord with my invention.

As pointed out above, one of the primary advantages of electron beam addressed memory systems is the ability to focus an electron beam to a very small diameter, in the order of 100 A or less. As a result, it is possible to provide data storage sites of only a few hundred Angstrorns, if desired. In accord with my invention and by way of example, assume that it is desired to provide an electron beam diameter of 0.1 microns 1,000 Angstroms) for accessing storage sites of less than 0.5 microns. For a given beam diameter, d, the maximum achievable beam current, I, in a 50 percent current diameter spot, d, at a given brightness, ,B, and spherical aberration of the matrix lens, C,, the following relationship has been found to exist:

max BIG? From this equation it can be seen that for a given spot size and beam brightness, the current is maximized by minimizing C Therefore, it is desirable to minimize C, for the matrix electron lens 23. If V and V,; are the voltages on lens plates 24 and 25, respectively, with respect to ground potential, and V is the voltage on the electron source, the accelerating voltage ratio V for the matrix electron lens is related to the lens voltages in the following manner:

V 1. x/ VM x) If the lenslets themselves are separated from each other by a distance S, have a diameter D and a radius R,'the optical properties of the lens are determined by the lens dimensions and the accelerating ratio V. In general, focal length and spherical aberration both decrease as the acceleration ratio increases for given lens dimensions. However, when the acceleration ratio is increased, the field strength, E, between the lens plate also increases. Additionally, as the acceleration ratio increases, the beam energy increases in the coarse deflection region. Accordingly, coarse beam deflection becomes more and more difficult as the acceleration ratio V increases. Therefore, it is necessary to maximize E, while avoiding voltage breakdown between the lens plates 24 and 25 but not increasing V and more than, necessary in order to avoid coarse deflection problems.

Assuming that it is desirable to operate the target structure at ground potential, V, 0 volts. Further, let it be assumed that V,, the voltage on the electron source 11 be minus 4.0 kv. Also, let it be assumed that the voltage on the lens plate 25 by 1.0 kv to provide sufficient acceleration to the secondary emission electrons. Using these values, it is then possible to select values for the acceleration ratio V, the spacing between the lens plates S, and the voltage on the lens plate 24. Table I illustrates these values for increasing field strengths E, of 5 X 10 V/cm, 10 V/cm and 2 X 10" V/cm.

From the foregoing information contained in Table I, the spherical aberration C in inches for the matrix electron lens can be plotted against lenslet radius R in mils, as shown in FIG. 4. From these curves, it is possible to select-operating conditions which are below the maximum spherical aberration C, and within the brightness capabilities of the electron source. For example, two possible operating conditions are S 60 mils, R 15 mils and E 10 V/cm and S 30 mils, R=7.5 and E=2 10 V/cm.

From Table I above, it can be seen that V must be 16 kv. Therefore, with V 4.0 kv, the electrons in the coarse deflection region have 20 kev energy. Further, it can be shown that the magnetic field required of the solenoid is approximately 200 gauss and that the coarse deflection can be achieved with gauss and the fine deflection achieved with only 3 gauss. These values permit the electron beam to be deflected over the target surface with the degree of accuracy required of a storage target having storage sites less than 0.5 microns. As a result, storage targets having storage densities in excess of 10 storage sites per square inch are readily provided.

To recapitulate operation of the apparatus shown schmatically in FIG. 1, the electron beam 12 is turned on whenever it is desired to read or write data, by deflecting the electron beam into the region enclosed by the coarse deflection assembly 20. The beam is then coarsely deflected by the coarse deflection coils 21 and 22 such that the beam is directed to a specific lenslet 26 in the matrix electron lens 23. This deflection may, for example, be controlled by voltages from a digitalto-analog converter in response to signals furnished by a computer;

The electron beam is then collimated by passage through the matrix electron lens 23 and enters the fine deflection region under the control of the fine deflection assembly 30. Deflection of the electron beam within this region is again controlled, for example, by digital-to-analog converters in response to output signals from the computer. The electron beam is then directed to a specific storage site on the storage target 13.

Where the target structure 13' comprises a semiconductor wafer having a radiation absorbing region coated over its incident radiation receiving surface,

such as is described in US. Pat. No. 3,550,094, the incident electron beam is used to write information on the target structure by 'causing the target to absorb radiation in its radiation absorbing coating. By designating a storage site which has received sufficient radiation to produce absorption in the radiation absorbing coating'as a logic 1, and storage sites which have received no such radiation, a logic 0, digital information may be stored in the target structure.

Readout of the stored information may be achieved in substantially a similar manner by deflecting the electron beam to a specific storage site, in the manner described above, and then monitoring the secondary electron emission from that particular storage site. More specifically, and continuing with the illustration wherein the storage target is a semiconductor wafer having a pattern of radiation absorbing regions thereon, during readout, the electron beam impinging on a storage site produces secondary emission of electrons at a rate which differs in accord with whether or not a storage site was previously written on by the write electron beam. The secondary emission electrons are focussed by the fine deflection assembly 30 back to the matrix electron lens 23 along a trajectory substantially similar to that of the primary electron beam 12. The return beams of secondary emission electrons are then detected by the detector 35. Since the number of detected electrons is proportional to the number emitted from the surface of the memory target, it is possible to read out the information stored on the storage target without destroying the stored information.

Suitable means for operating a data storage system in accord with our invention are described in the foregoing patents and hence specific circuitry to achieve the foregoing read and write functions are not disclosed herein. It should also be apparent to those skilled in the art that whereas the invention has been described with respect to readout by means of secondary emission electrons, obviously where the information stored on a storage media is in the form of a latent electrostatic image, readout of the stored information may be achieved by reflected or turned around electrons reflected from the surface of the storage media. In this case, the return beam of electrons are not secondary emission electrons, but rather reflected or turned around primary electrons. However, the optical system in accord with out invention readily accommodates either secondary emission electrons or reflected electrons which are still detected as described above.

From the above description, it can be readily appreciated that the present invention provides a new and improved write-read electron apparatus and method making possible readout from a storage target in a nondestructive manner with an electron beam by detecting It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electron beam-addressable memory syste comprising:

an electron source for producing a beam of electrons;

a 'data storage target disposed in the path of said beam for storing information thereon;

a matrix electron lens disposed in the path of said beam and positioned adjacent to said storage target, said lens comprising a pair of spaced parallel lens plates each having a plurality of lenslets therein for focussing said beam on said target;

coarse deflection means positioned between said electron source and said lens for directing said beam to a specific lenslet;

fine deflection means. positioned between said lens and said storage target for directing said beam to a specific point of impingement on said storage target, said target emitting secondary electrons in response to said beam impinging on said target;

means for returning said secondary emission electrons along a path substantially similar to said electron beam; and

means for detecting said secondary emission electrons and providing an output signal proportional to the detected secondary emission electrons.

2. The memory system of claim 1 wherein information stored on said target is'representative of digital information and said secondary emission electrons are emitted in proportional response to said stored information.

3. The memory system of claim 1 further comprising:

means establishing a magnetic field along the axis of said electron beam, said magnetic field relaying said focussed electron beam existing from said lenslet to said target with a single node.

4. The memory system of claim 3 further comprising:

means biasing said parallel lens plates to produce a decelerating electric field for decelerating and focussing said electron beam in front of said lenslet.

5. The memory system of claim 4 wherein said secondary electrons are directed to said detecting means under the influence of said magnetic field and said electric field.

6. The memory system of claim 5 wherein said secondary electrons emitted from the target surface are directed to a lenslet of said matrix electron lens with a multinode focus of varying pitch.

7. The memory system of claim 3 wherein said electron beam impinges on said target substantially normal thereto. I

8. The memory system of claim 5 where said means for detecting comprises a semiconductor detector positioned along the path of said electron beam between said coarse deflection means and said electron source, said detector having an aperture therein for the passage of said electron beam therethrough.

9. The memory system of claim 4 whereinsaid means biasing said parallel lens plates produces an accelerating ratio in the range of 3 to 5.

10. A method of reading out information stored on a target surface with an electron beam, said method comprising the steps of forming a beam of electrons;

focussing said beam of electrons on said target, said target emitting secondary electrons in response to said beam of electrons;

returning said secondary emission electrons along a path substantially similar to said electron beam; and

detecting said secondary electrons along said path.

11. The method of claim 10 wherein the step of focussing comprises:

coarsely deflecting said beam of electrons to a lenslet of a matrix electron lens;

decelerating said beam of electrons with said matrix electron lens; and

finely deflecting said beam of electrons to a specific point of impingement on said target surface.

12. The method of claim 11 wherein said secondary emission electrons are emitted in proportional response to information stored on said target.

13. The method of claim 12 wherein the step of reemission electrons, 

1. An electron beam-addressable memory system comprising: an electron source for producing a beam of electrons; a data storage target disposed in the path of said beam for storing information thereon; a matrix electron lens disposed in the path of said beam and positioned adjacent to said storage target, said lens comprising a pair of spaced parallel lens plates each having a plurality of lenslets therein for focussing said beam on said target; coarse deflection means positioned between said electron source and said lens for directing said beam to a specific lenslet; fine deflection means positioned between said lens and said storage target for directing said beam to a specific point of impingement on said storage target, said target emitting secondary electrons in response to said beam impinging on said target; means for returning said secondary emission electrons along a path substantially similar to said electron beam; and means for detecting said secondary emission electrons and providing an output signal proportional to the detected secondary emission electrons.
 2. The memory system of claim 1 wherein information stored on said target is representative of digital information and said secondary emission electrons are emitted in proportional response to said stored information.
 3. The memory system of claim 1 further comprising: means establishing a magnetic field along the axis of said electron beam, said magnetic field relaying said focussed electron beam existing from said lenslet to said target with a single node.
 4. The memory system of claim 3 further comprising: means biasing said parallel lens plates to produce a decelerating electric field for decelerating and focussing said electron beam in front of said lenslet.
 5. The memory system of claim 4 wherein said secondary electrons are directed to said detecting means under the influence of said magnetic field and said electric field.
 6. The memory system of claim 5 wherein said secondary electrons emitted from the target surface are directed to a lenslet of said matrix electron lens with a multinode focus of varying pitch.
 7. The memory system of claim 3 wherein said electron beam impinges on said target substantially normal thereto.
 8. The memory system of claim 5 wheRe said means for detecting comprises a semiconductor detector positioned along the path of said electron beam between said coarse deflection means and said electron source, said detector having an aperture therein for the passage of said electron beam therethrough.
 9. The memory system of claim 4 wherein said means biasing said parallel lens plates produces an accelerating ratio in the range of 3 to
 5. 10. A method of reading out information stored on a target surface with an electron beam, said method comprising the steps of forming a beam of electrons; focussing said beam of electrons on said target, said target emitting secondary electrons in response to said beam of electrons; returning said secondary emission electrons along a path substantially similar to said electron beam; and detecting said secondary electrons along said path.
 11. The method of claim 10 wherein the step of focussing comprises: coarsely deflecting said beam of electrons to a lenslet of a matrix electron lens; decelerating said beam of electrons with said matrix electron lens; and finely deflecting said beam of electrons to a specific point of impingement on said target surface.
 12. The method of claim 11 wherein said secondary emission electrons are emitted in proportional response to information stored on said target.
 13. The method of claim 12 wherein the step of returning said secondary emission electrons comprises: establishing a magnetic field along the axis of said electron beam for directing said secondary emission electrons to said matrix electron lens with a multinode focus of varying pitch.
 14. The method of claim 13 further comprising: biasing said matrix electron lens to produce a decelerating electric field for said beam of electrons and an accelerating electric field for said secondary emission electrons. 